**1. Introduction**

Bioreactors are closed systems that allow the cultivation of microbial, animal, and human cells in a controlled, aseptic environment. The simplest bioreactors are not instrumentalised (e.g., shake flasks) and dominate in mL-scale process development. In contrast, bench-top (10 L working volume), pilot (≥50 L working volume), and

industrial-scale (working volumes of cubic metres) bioreactors are typically equipped with sensors, which allow regulation and adjustment of the cultivation process. At this time, the primary goal of such cultivation is the production of inocula, and bio-, cell-, or gene therapeutics. The bioreactor type and its operation parameters influence the growth and production behaviour of the production organism used. For example, the often-used microorganism *Eschericha coli* has high oxygen demands [1, 2] and a need for short mixing times [3] due to its fast metabolism. In addition, microorganisms such as *E. coli* are extremely robust thanks to their cell walls and small size. Therefore, the bioreactor can be operated with a high specific power input. Biopharmaceutical production with *E. coli* has a long tradition and dates back to 1982 when the production of insulin was approved by the Food and Drug Administration in the USA [4].

Today, most biopharmaceuticals, such as monoclonal antibodies, therapeutic hormones and many vaccines, are produced using mammalian cells, with Chinese hamster ovary (CHO) cells being the most frequently used [5]. Furthermore, human embryonic kidney (HEK) cells, Madin-Darby canine kidney (MDCK) cells, murine myeloma cells (NS0), and baby hamster kidney (BHK) cells are used [6]. The advantage of mammalian cells is that they can produce more complex molecules and can also perform glycosylation, which makes them even more attractive to use [7]. Furthermore, mammalian cells generally do not require as much oxygen as *E. coli* [1, 8]. However, they are larger and lack a cell wall, which makes them much more sensitive to fluid dynamic stress, a factor that has to be taken into account when selecting the bioreactor system and its operation parameters [9, 10].

In addition to mammalian cells and microorganisms, plant cells, insect cells and, more recently, stem cells are also used in biotechnological manufacturing processes. These cells are either continuous cells (unlimited life span) or primary cells (limited life span) and grow adherently (they need a surface to attach to, such as planar plastic surface, membranes or microcarriers) or in suspension.

Bioreactors can be classified in different ways. If the reusability of the cultivation container is taken into account, a classification into single-use and reusable systems can be made [11, 12]. If the type of mass transfer is taken into account, they can be categorised into static and dynamic systems, the latter of which can be further divided into mechanically, pneumatically and hydraulically driven bioreactors, depending on the type of power input [13, 14]. Mechanically driven systems, which are either stirred, orbitally shaken, or wave-mixed, predominate [13], the last of which are exclusively available as single-use variants. Single-use bioreactors have become very popular over the past 20 years [15]. The cultivation containers (rigid plastic vessels or flexible bags) are pre-sterilised, can be used immediately, and are disposed of after the cultivation has been completed. This makes the cultivation safer, helps to save time, may reduce costs, and often contributes to a lower impact on the environment despite the plastic waste that is generated. Efforts by manufacturers of single-use bioreactors have helped make them more user-friendly, for example, by replacing weak points such as the film material of the first and second bioreactor generation with improved ones that are less problematic in terms of leachables and extractables [16]. Currently, single-use bioreactors up to 6 m<sup>3</sup> working volume are available on the market [15].

Stirred bioreactors (**Figure 1a**) are characterised by a mostly cylindrical vessel containing a stirring shaft with one or more stirrers. The dimensioning of the vessel as well as the type, number and positioning of the stirrer(s), which introduce power into the system, differ depending on the application. The oxygen required by the cells is added via the fluid surface or by active gassing with a sparger, depending on the application. Stirred bioreactors range from the mL-scale in research and development *Computational Fluid Dynamics for Advanced Characterisation of Bioreactors Used… DOI: http://dx.doi.org/10.5772/intechopen.109848*

#### **Figure 1.**

*Computer-generated images (CGI) of common mechanically driven bioreactor systems. a) Stirred bioreactor where the power is brought into the system by rotation of the stirring shaft and the stirrer (rotation marked by green arrow). b) Shake flask, which is an orbitally shaken bioreactor. Here the power is brought into the system by the orbitally shaking of the platform (movement of the platform marked by green arrow). c) Wave-mixed bioreactor, in which the power is brought in by the back and forth tilting of the bioreactor (rotation around tilting axis marked by green arrow).*

to more than 100 m<sup>3</sup> in production processes [12]. In orbitally shaken systems, such as the shake flasks (**Figure 1b**), power is introduced into the system through the walls of the vessel by moving the entire system orbitally on a platform, and oxygen is typically only introduced via the free surface. Wave-mixed systems (**Figure 1c**) are also

surface-aerated however, in these systems power is introduced by tilting the cultivation bag to create one or two degrees of freedom (DOF) of motion, resulting in an undulation in the fluid inside the bioreactor.

Bioreactors should be both, technically characterised and biologically qualified. Details on biological qualification of bioreactors are described by Schirmer et al. [17, 18]. Technical or so-called classical process engineering characterisation includes the determination of specific power input, mixing or residence time, and oxygen transfer [19]. If adherent cells such as mesenchymal and induced pluripotent stem cells are grown on microcarriers, investigation of the suspension behaviour can also be beneficial [20]. Characterisation is typically performed in the laboratory by means of experiments (Section 4). However, such characterisation is resource-intensive (financial and labour costs). Moreover, only bioreactor systems that already physically exist can be characterised. However, with the help of computational fluid dynamics (CFD), it is possible to eliminate some of these disadvantages, making it is possible to characterise and optimise digital systems prior to construction. This allows expensive production stops to be avoided, and more complex investigations to be performed. Nevertheless, there are certain disadvantages associated with the use of CFD. Well-trained personnel and a lot of computing power are required. Validation is necessary to obtain reliable results, since CFD is only a model of the real world that is limited by a finite degree of accuracy. In this chapter, process parameters and their determination by CFD are described and discussed. In addition, possible hardware and software solutions are also presented as well as validation methods.
